Coordination of vehicle actuators during firing fraction transitions

ABSTRACT

A variety of methods and arrangements are described for controlling transitions between firing fractions during skip fire and potentially variable displacement operation of an engine. In general, actuator first transition strategies are described in which an actuator position (e.g., cam phase, TCC slip, etc.) is changed to, or close to a target position before i corresponding firing fraction change is implemented. When the actuator change associated with a desired firing fraction change is relatively large, the firing fraction change is divided into a series of two or more firing fraction change steps. A number of intermediate target selection schemes are described as well.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.62/353,674, filed on Jun. 23, 2016, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to methods and arrangements forcontrolling vehicle actuators during firing fraction transitions in askip fire controlled engine.

BACKGROUND

Modern vehicles utilize many actuators, controlling various aspects ofvehicle operation. Many of these actuators control engine operation,such as throttle, cam phase, fuel injection, and spark timing. Otheractuators control delivery of the engine torque to a vehicle's wheels,such as a torque converter or a transmission. Operation of theseactuators must be coordinated to achieve acceptable vehicle performance.In particular it is desirable to control a vehicle to provide optimumfuel efficiency with acceptable NVH (noise, vibration, harshness)performance.

Fuel efficiency of many types of internal combustion engines can besubstantially improved by varying the displacement of the engine. Thisallows for the full torque to be available when required, yet cansignificantly reduce pumping losses and improve thermodynamic efficiencythrough the use of a smaller displacement when full torque is notrequired. The most common method of varying the displacement today isdeactivating a group of cylinders substantially simultaneously. In thisapproach no fuel is delivered to the deactivated cylinders and theirassociated intake and exhaust valves are kept closed as long as thecylinders remain deactivated.

Another engine control approach that varies the effective displacementof an engine is referred to as “skip fire” engine control. In general,skip fire engine control contemplates selectively skipping the firing ofcertain cylinders during selected firing opportunities. Thus, aparticular cylinder may be fired during one engine cycle and then may beskipped during the next engine cycle and then selectively skipped orfired during the next. Skip fire engine operation is distinguished fromconventional variable displacement engine control in which a designatedset of cylinders are deactivated substantially simultaneously and remaindeactivated as long as the engine remains in the same variabledisplacement mode. Thus, the sequence of specific cylinders firings willalways be exactly the same for each engine cycle during operation in avariable displacement mode (so long as the engine remains in the samedisplacement mode), whereas that is often not the case during skip fireoperation. For example, an 8 cylinder variable displacement engine maydeactivate half of the cylinders (i.e. 4 cylinders) so that it isoperating using only the remaining 4 cylinders. Commercially availablevariable displacement engines available today typically support only twoor at most three fixed displacement modes.

In general, skip fire engine operation facilitates finer control of theeffective engine displacement than is possible using a conventionalvariable displacement approach. For example, firing every third cylinderin a 4 cylinder engine would provide an effective displacement of ⅓^(rd)of the full engine displacement, which is a fractional displacement thatis not obtainable by simply deactivating a set of cylinders.Conceptually, virtually any effective displacement can be obtained usingskip fire control, although in practice most implementations restrictoperation to a set of available firing fractions, sequences or patterns.One of the Applicants, Tula Technology has filed a number of patentsdescribing various approaches to skip fire control.

Many skip fire controllers are arranged to provide a set of availablefiring patterns, sequences or firing fractions. In some circumstancesthe set of available firing patterns or fractions will vary as afunction of various operating parameters such as engine load, enginespeed and transmission gear. Typically the available firing patterns areselected, in part, based on their NVH characteristics. Transitionsbetween firing fraction levels must be managed to avoid unacceptable NVHduring the transition. In particular, changes in the firing fractionmust be coordinated with other engine actuators to achieve smooth firingfraction transitions.

Many internal combustion engines incorporate a cam phaser to adjust acam angle or phase relative to the crankshaft. Adjusting the cam phasevaries the relative timing of the opening and closing of the intakeand/or exhaust valves relative to top dead center (TDC) or some othercrankshaft reference point. The cam phase impacts both the cylinder massair charge (MAC) and the amount of residual exhaust gases left in thecylinder from the preceding cylinder working cycle.

Some engine valve trains utilize a single camshaft to actuate bothintake and exhaust valves, while others utilize separate camshafts forthe intake and exhaust valves. Still other engines have cylindersarranged in banks with single or dual camshafts dedicated to each bank.When a cam phaser is used in conjunction with a camshaft that actuatesboth intake and exhaust valves, then cam phase adjustments will affectboth the intake and exhaust strokes. When dual camshafts thatindependently actuate intake and exhaust valves are used, then theintake and exhaust valve timings may be independently varied.

The cam phase can be set to provide optimum fuel efficiency (or otherdesired characteristics), however the optimal cam phase varies as afunction of the engine speed and the cylinder load. Therefore, the fuelefficiency of an engine may generally be improved by varying the camphase based on the engine operating conditions.

In addition to cam phase there are other actuators and control systemsin modern vehicles that impact fuel efficiency and occupant comfort. Onesuch system is control of the torque converter slip. The torqueconverter transfers motive power between the vehicle's engine andwheels. Torque converter slip indicates the difference in rotationalvelocity between the input, engine side, of the torque converter and theoutput, wheel side, of the torque converter. For fuel efficiency it isdesirable to minimize or eliminate slip; however, insufficient slip willcause unacceptable NVH and compromise a vehicle's drivability.

There is need for control methods that coordinate changes in the firingfraction with adjustment of other vehicle actuators, such as cam phaseand torque converter slip. The present application describes approachesfor combining control of various vehicle actuators with skip fireoperation to provide fuel efficient transitions between different firingpatterns, sequences or firing fractions. In particular, control of camphasing and torque converter slip are described, but the conceptspresented herein are applicable to a broad range of vehicle actuators.

SUMMARY

A variety of methods, controllers and arrangements are described formanaging transitions between firing fractions during skip fire orvariable displacement operation of an engine. In general, drivetrainslip first transition strategies are described in which increases in thedrivetrain slip are changed to, or close to a target drivetrain slipbefore a corresponding firing fraction change is implemented. When thedrivetrain slip change associated with a desired firing fraction changeis relatively large, the firing fraction change may be divided into aseries of two or more firing fraction change steps. A number ofintermediate target selection schemes are described as well.

In one aspect, methods and controllers suitable for managing firingfraction transitions are disclosed. When a request is made to transitionto a firing fraction having a higher slip transition threshold than thecurrent operating slip, a transition towards the requested drivelineslip is initiated. When appropriate (optionally), one or moreintermediate target firing fraction may be identified and the firingfraction transition may be divided into multiple stages. The targetfiring fraction (i) is selected from a set of available firing fractionscapable of delivering a requested engine output, and (ii) has anassociated target drivetrain slip transition threshold that is less theslip transition threshold associated with the requested firing fraction.Each firing fraction transition is constrained to only occur when anactual driveline slip is at least as high as the associated drivetrainslip transition threshold.

In some embodiments, when the requested firing fraction is higher thanthe original firing fraction, the selected target firing fraction is thelowest available firing fraction capable of delivering the requestedengine output that has an associated driveline slip transition thresholdthat is not more than the then current drivetrain slip. This may resultin a transitory transition to a target firing fraction that is higherthan the requested firing fraction

When the requested firing fraction is lower than the first firingfraction, the selected target firing fraction is preferably anintermediate firing fraction between the first and second firingfractions. In some embodiments, the selected target firing fraction isthe lowest available firing fraction capable of delivering the requestedengine output that has an associated driveline slip transition thresholdthat is not more than the original slip when such an intermediate firingfraction exists.

In some embodiments, the driveline slip is imparted by a torqueconverter clutch (TCC).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and the advantages thereof, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a functional block diagram of a skip fire controller having atransition management control unit.

FIG. 2A illustrates a timing diagram for an exemplary cam firsttransition control method.

FIG. 2B illustrates a timing diagram for an exemplary concurrenttransition control method.

FIG. 2C illustrates a timing diagram for an exemplary cam firsttransition strategy using intermediate firing fractions.

FIG. 3 is a flow chart illustrating a cam first firing fractiontransition management scheme in accordance with a nonexclusiveembodiment.

FIG. 4 is a flow chart illustrating a method of selecting a next targetfiring fraction in accordance with a nonexclusive embodiment.

FIG. 5 is a flow chart illustrating a drivetrain slip first firingfraction transition management scheme in accordance with anothernonexclusive embodiment.

FIG. 6 is a flow chart illustrating a method of selecting a next targetfiring fraction in the drivetrain slip first firing fraction transitionmanagement scheme of FIG. 5.

In the drawings, like reference numerals are sometimes used to designatelike structural elements. It should also be appreciated that thedepictions in the figures are diagrammatic and not to scale.

DETAILED DESCRIPTION

When a transition is made between different firing fractions (orvariable displacement states) there is typically a corresponding need ordesire to change certain engine or vehicle operating parameters such asair charge, fuel charge, spark timing, drivetrain slip, etc. This isbecause at any particular firing density, there will be associatedoperating parameters that are appropriate to most efficiently deliverthe desired engine output while maintaining desired performance andpassenger comfort standards. Therefore, when a change is made in thefiring density, it is typically desirable to concurrently adjust one ormore selected engine operating parameters and/or drivetraincharacteristics so that the desired engine output and vehicleperformance is maintained both throughout the transition and at the newfiring fraction. Without such an adjustment, operating at the sameengine settings would typically result in the generation of more torquethan desired when the firing density is increased, and less torque thandesired when the firing density is reduced.

From a control standpoint, the firing density can be changed veryquickly by simply altering the selection of the specific cylinders to befired—however corresponding changes in the air charge tend to berealized more slowly due to the latencies inherent in changing the camphase, filling or emptying the intake manifold, etc. This isparticularly noticeable when the desired firing fraction changessignificantly, as for example when transitioning from a firing fractionof 1 to ½ or from ⅔ to ⅓, which requires correspondingly large changesin air charge. Generally, any mismatch between the firing density andthe targeted cylinder air charge during a transition, will result in alow frequency torque disturbance (unless otherwise compensated for),which may be perceived as NVH. If the mismatch would result in a torquesurge, then the spark timing can be retarded to maintain the desiredtorque. However, an undesirable side effect of retarding spark to reduceengine output is that retarding spark will generally reduce fuelefficiency. Also, excessive spark retard could lead to misfires furtherreducing efficiency and potentially adversely affecting the engineperformance. Modern engine control often limits the amount of sparkretard to ensure proper combustion.

Tula Technology has previously described a variety of techniques fortransitioning between different firing fractions. By way of example,various transition control schemes are described in U.S. Pat. No.9,086,020 (P011A) and U.S. Pat. No. 9,086,020 (P029) and applicationSer. Nos. 14/857,371 (P041), and 62/296,451 (P054P) each of which isincorporated herein by reference. Although these and other existingtransition schemes work well, there are continuing efforts to providecontrol schemes and controllers for transitioning between differentfiring patterns or different firing fractions that work well in avariety of different situations.

In general, it tends to be more fuel efficient to vary (and particularlyreduce) the air charge using cam control rather than throttling whenpossible, since varying the air charge using cam control doesn't havethe same level of pumping losses as throttling. The most common exampleof cam based control of the air charge is the use of a cam phaser tocontrol the phase of the camshaft relative to the crankshaft. However,other types of cam-based control can also be used to control the aircharge, such as valve lift, multiple cams, dual cams, etc. when theengine is equipped with appropriate hardware. In engines equipped withcam phase control, some engine control schemes conceptually utilize camphase as the first mechanism for varying air charge. However a feature(and potential drawback) of cam phase control is that camshaft phaseadjustment tends to proceed relatively slowly. For example, conventionalcam phasers tend to have slew rates of less than 200 degrees/second andsome operate significantly slower than that, as for example, in thevicinity of 50 degrees/second. Since a cam phaser may have the authorityto change the cam phase by 50 to 60 degrees, it may take tens of firingopportunities for the cam phaser to realize a large commanded change incam phase. In some commercial implementations, a worst case cam phasetransition time can be on the order of 1.5 seconds. Large cam phasetransitions tend to be slow compared to the ability to implement firingfraction changes and compared to other engine control actuators such asspark timing, and throttle control of the intake manifold pressure.

One way to implement a transition from a first operating state (e.g. afirst firing fraction at a first cam phase and air charge) to asecond/target operating state having a lower firing fraction (e.g., asecond/target firing fraction at a second/target cam and air charge) isto first transition the cam phase while using other control actuatorssuch as spark timing and throttle to help ensure that the desired enginetorque output is attained through the transition. Then, after the targetcam phase has been attained or is within some range of the target camvalue, the change in firing fraction can be implemented—again usingother control parameters such as spark and throttle to manage the torqueoutput during the transition. This approach can be referred to as a camfirst transition approach. Although the cam first transition approachfits nicely with air control schemes that focus on cam phase control toadjust the air charge, it suffers undesirable efficiency losses duringmany transitions—particularly when the requested cam phase adjustment isrelatively large. This is because the average intake manifold pressureduring the transition will typically be lower than it would be if thefiring fraction were free to transition immediately, increasing pumpinglosses.

Applicants have determined that the fuel efficiency associated with camfirst transition control during reductions in firing fraction can besignificantly improved by breaking relatively larger firing fractiontransitions into a set of two or more smaller transitions. In thisapproach, a cam first transition is initially made to an intermediatefiring fraction. Once the actual cam phase gets close to, or reaches atarget cam phase associated with the intermediate firing fraction, thefiring fraction is adjusted to the intermediate firing fraction. A camfirst transition is then made to the next intermediate firing fractionor to the final desired firing fraction, as appropriate. Theintermediate targets allow the firing fraction to be reduced atintermediate points in the transition, providing a higher average intakemanifold pressure which results in improved fuel efficiency during theoverall transition. This control strategy is referred to herein as astaged, cam first transition strategy.

Tula Technology has previously described a variety of skip firecontrollers. A suitable skip fire controller 10 is functionallyillustrated in FIG. 1. The illustrated skip fire controller 10 includesa torque calculator 20, a firing fraction and power train settingsdetermining unit 30, a transition adjustment unit 40, and a firingtiming determination unit 50. For the purposes of illustration, skipfire controller 10 is shown separately from powertrain control unit orengine control unit (ECU) 70 which implements the commanded firings andprovides the detailed component controls. However, it should beappreciated that in many embodiments the functionality of the skip firecontroller 10 may be incorporated into the ECU 70. Indeed incorporationof the skip fire controller into an ECU or power train control unit isexpected to be the most common implementation.

The torque calculator 20 is arranged to determine the desired enginetorque at any given time based on a number of inputs. The torquecalculator outputs a requested torque 21 to the firing fraction andpower train settings determining unit 30. The firing fraction and powertrain settings determining unit 30 is arranged to determine a firingfraction that is suitable for delivering the desired torque based on thecurrent operating conditions and outputs a desired operational firingfraction 33 that is appropriate for delivering the desired torque. Unit30 also determines selected engine operating settings (e.g., manifoldpressure 31, cam timing 32, torque converter slip 35, etc.) that areappropriate to deliver the desired torque at the designated firingfraction.

In many implementations, the firing fraction and engine and power trainsettings determining unit selects between a set of predefined firingfractions which are determined to have relatively good NVHcharacteristics. In such embodiments, there are periodically transitionsbetween desired operational firing fractions. It has been observed thattransitions between operational firing fractions can be a source ofundesirable NVH. Transition adjustment unit 40 is arranged to adjust thecommanded firing fraction and certain engine settings (e.g., camshaftphase, throttle plate position, intake manifold pressure, torqueconverter slip) during transitions in a manner that helps mitigate someof the transition associated NVH.

The firing timing determining unit 50 is responsible for determining thespecific timing of firings to deliver the desired firing fraction. Thefiring sequence can be determined using any suitable approach. In somepreferred implementations, the firing decisions are made dynamically onan individual firing opportunity by firing opportunity basis, whichallows desired changes to be implemented very quickly. A variety offiring timing determining units that are well suited for determiningappropriate firing sequences based on a potentially time varyingrequested firing fraction or engine output have been previouslydescribed by Tula. Many such firing timing determining units are basedon a sigma delta converter, which is well suited for making firingdecisions on a firing opportunity by firing opportunity basis. In otherimplementations, pattern generators or predefined patterns may be usedto facilitate delivery of the desired firing fraction.

The torque calculator 20 receives a number of inputs that may influenceor dictate the desired engine torque at any time. In automotiveapplications, one of the primary inputs to the torque calculator is theaccelerator pedal position (APP) signal 24 which indicates the positionof the accelerator pedal. In some implementations the accelerator pedalposition signal is received directly from an accelerator pedal positionsensor (not shown) while in others an optional preprocessor 22 maymodify the accelerator pedal signal prior to delivery to the skip firecontroller 10. Other primary inputs may come from other functionalblocks such as a cruise controller (CCS command 26), the transmissioncontroller (AT command 27), a traction control unit (TCU command 28),etc. There are also a number of factors such as engine speed that mayinfluence the torque calculation. When such factors are utilized in thetorque calculations, the appropriate inputs, such as engine speed (RPMsignal 29) are also provided or are obtainable by the torque calculatoras necessary.

Further, in some embodiments, it may be desirable to account forenergy/torque losses in the drive train and/or the energy/torquerequired to drive engine accessories, such as the air conditioner,alternator/generator, power steering pump, water pumps, vacuum pumpsand/or any combination of these and other components. In suchembodiments, the torque calculator may be arranged to either calculatesuch values or to receive an indication of the associated losses so thatthey can be appropriately considered during the desired torquecalculation.

The nature of the torque calculation will vary with the operationalstate of the vehicle. For example, during normal operation, the desiredtorque may be based primarily on the driver's input, which may bereflected by the accelerator pedal position signal 24. When operatingunder cruise control, the desired torque may be based primarily on theinput from a cruise controller. When a transmission shift is imminent, atransmission shifting torque calculation may be used to determine thedesired torque during the shifting operation. When a traction controlleror the like indicates a potential loss of traction event, a tractioncontrol algorithm may be used to determine the desired torque asappropriate to handle the event. In some circumstances, depression of abrake pedal may invoke specific engine torque control. When other eventsoccur that require measured control of the engine output, appropriatecontrol algorithms or logic may be used to determine the desired torquethroughout such events. In any of these situations, the required torquedeterminations may be made in any manner deemed appropriate for theparticular situation. For example, the appropriate torque determinationsmay be made algorithmically, using lookup tables based on currentoperating parameters, using appropriate logic, using set values, usingstored profiles, using any combinations of the foregoing and/or usingany other suitable approach. The torque calculations for specificapplications may be made by the torque calculator itself, or may be madeby other components (within or outside the ECU) and simply reported tothe torque calculator for implementation.

The firing fraction and power train settings determining unit 30receives requested torque signal 21 from the torque calculator 20 andother inputs such as engine speed 29 and various power train operatingparameters and/or environmental conditions that are useful indetermining an appropriate operational firing fraction 33 to deliver therequested torque under the current conditions. Power train parametersinclude, but are not limited to, throttle position, cam phase angle,fuel injection timing, spark timing, torque converter slip, transmissiongear, etc. The firing fraction is indicative of the fraction orpercentage of firings that are to be used to deliver the desired output.In some embodiments the firing fraction may be considered as an analoginput into a sigma-delta converter. Often, the firing fractiondetermining unit will be constrained to a limited set of availablefiring fractions, patterns or sequences that have been selected based atleast in part on their relatively more desirable NVH characteristics(collectively sometimes referred to herein generically as the set ofavailable firing fractions). There are a number of factors that mayinfluence the set of available firing fractions. These typically includethe requested torque, cylinder load, engine speed (e.g. RPM), vehiclespeed and current transmission gear. They may potentially also includevarious environmental conditions such as ambient pressure or temperatureand/or other selected power train parameters. The firing fractiondetermining aspect of unit 30 is arranged to select the desiredoperational firing fraction 33 based on such factors and/or any otherfactors that the skip fire controller designer may consider important.By way of example, a few suitable firing fraction determining units aredescribed in application Ser. Nos. 13/654,244; 13/654,248, 13/963,686,14/638,908, and 62/296,451, each of which are incorporated herein byreference.

The number of available firing fractions/patterns and the operatingconditions during which they may be used may be widely varied based onvarious design goals and NVH considerations. In one particular example,the firing fraction determining unit may be arranged to limit availablefiring fractions to a set of 29 possible operational firingfractions—each of which is a fraction having a denominator of 9 orless—i.e., 0, 1/9, ⅛, 1/7, ⅙, ⅕, 2/9, ¼, 2/7, ⅓, ⅜, ⅖, 3/7, 4/9, ½, 5/9,4/7, ⅗, ⅝, ⅔, 5/7, ¾, 7/9, ⅘, ⅚, 6/7, ⅞, 8/9 and 1. However, at certain(indeed most) operation conditions, the set of available firing fractionmay be reduced and sometimes the available set is greatly reduced. Ingeneral, the set of available firing fractions tends to be smaller inlower gears and at lower engine speeds. For example, there may beoperating ranges (e.g. near idle and/or in first gear) where the set ofavailable firing fractions is limited to just two availablefractions—(e.g., ½ or 1) or to just 4 possible firing fractions—e.g., ⅓,½, ⅔ and 1. Of course, in other embodiments, the permissible firingfractions/patterns for different operating conditions may be widelyvaried.

When the available set of firing fractions is limited, various powertrain operating parameters such as mass air charge (MAC) and/or sparktiming will typically need to be varied to ensure that the actual engineoutput matches the desired output. In the embodiment illustrated in FIG.1, this functionality is incorporated into the power train settingscomponent of unit 30. In other embodiments, it can be implemented in theform of a power train parameter adjusting module (not shown) thatcooperates with a firing fraction calculator. Either way, the powertrain settings component of unit 30 or the power train parameteradjusting module determines selected power train parameters that areappropriate to ensure that the actual engine output substantially equalsthe requested engine output at the commanded firing fraction and thatthe wheels receive the desired brake torque. Torque converter slip maybe included in the determination of appropriate power train parameters,since increasing the torque converter slip will generally decrease theperceived NVH. Depending on the nature of the engine, the air charge canbe controlled in a number of ways. Most commonly, the air charge iscontrolled by controlling the intake manifold pressure and/or the camphase (when the engine has a cam phaser or other mechanism forcontrolling valve timing). However, when available, other mechanism suchas adjustable valve lifters, air pressure boosting devices liketurbochargers or superchargers, air dilution mechanism such as exhaustgas recirculation or other mechanisms can also be used to help adjustthe air charge. In the illustrated embodiment, the desired air charge isindicated in terms of a desired intake manifold pressure (MAP) 31 and adesired cam phase setting 32. Of course, when other components are usedto help regulate air charge, there may be indicated values for thosecomponents as well.

The firing timing determining module 50 is arranged to issue a sequenceof firing commands 52 that cause the engine to deliver the percentage offirings dictated by a commanded firing fraction 48. The firing timingdetermining module 50 may take a wide variety of different forms. By wayof example, sigma delta convertors work well as the firing timingdetermining module 50. A number of Tula's patents and patentapplications describe various suitable firing timing determiningmodules, including a wide variety of different sigma delta basedconverters that work well as the firing timing determining module. See,e.g., U.S. Pat. Nos. 7,577,511, 7,849,835, 7,886,715, 7,954,474,8,099,224, 8,131,445, 8,131,447, 8,839,766 and 9,200,587. The sequenceof firing commands (sometimes referred to as a drive pulse signal 52)outputted by the firing timing determining module 50 may be passed to anengine control unit (ECU) 70 or another module such as a combustioncontroller (not shown in FIG. 1) which orchestrates the actual firings.A significant advantage of using a sigma delta converter or an analogousstructure is that it inherently includes an accumulator function thattracks the portion of a firing that has been requested, but not yetdelivered. Such an arrangement helps smooth transitions by accountingfor the effects of previous fire/no fire decisions.

When a change in firing fraction is commanded by unit 30, it will often(indeed typically) be desirable to simultaneously command a change inthe cylinder mass air charge (MAC). As discussed above changes in theair charge tend to be realized more slowly than changes in firingfraction can be implemented due to the latencies inherent in filling oremptying the intake manifold and/or adjusting the cam phase. Transitionadjustment unit 40 is arranged to adjust the commanded firing fractionas well as various operational parameters such as commanded cam phaseand commanded manifold pressure during transitions in a manner thatmitigates unintended torque surges or dips during the transition. Thatis, the transition adjustment unit manages at least the target camphase, one or more other actuators that impact the air charge (e.g.throttle position), and the firing fractions during transitions betweencommanded firing fractions. It may also control other power trainparameters, such as torque converter slip.

FIGS. 2A, 2B, and 2C are exemplary timing diagrams depicting varioustransition control strategies. All cases assume that the transitionshould maintain a constant torque output through the transition,although this is not a requirement. For the purposes of illustration,all cases further assume a constant maximum cam slew rate of 60°/sec anda linear firing fraction slew rate of 0.004/msec. The initial and finalfiring fraction are ⅔ and ⅓ in all cases and the initial and final camphase angles are 50° and 15° in all cases. The cam phase angletransition threshold is ±3 degrees of crankshaft rotation angle for allfiring fraction levels. The cam angle ranges are depicted by dashedhorizontal lines in all FIGS. 2A, 2B, and 2C.

FIG. 2A illustrates an exemplary transition that utilizes a cam firsttransition approach. Changes in the cam phase angle 210 and the firingfraction 212 are depicted as a function of time. The transition beginsat 50 msec, denoted by line 202. When a command to change firingfraction is received, the cam phase angle 210 begins to change at itsmaximum slew rate, in this example 60°/sec. The firing fraction 212remains fixed at its initial value until the cam phase angle reaches oris within a defined range of its final target value. In this example,the final cam phase target 214 is 15° and the range 216 is within ±3° ofthe final cam phase. The cam phase angle 210 reaches the defined rangeof its target value at approximately 580 msec, denoted by line 204. Thefiring fraction 212 then begins to transition towards its target value,in this example ⅓. The firing fraction slew rate in this example is0.004/msec. If the engine is an 8 cylinder engine operating at 1500 thiscorresponds to a slew rate of 0.04 per firing opportunity. The change inthe firing fraction may be linear as described in co-pending applicationSer. No. 14/857,371 (P041); however, this is not a requirement. Itshould be appreciated that the firing fraction slew rate can be chosenbased on providing acceptable NVH performance during the transition andother concerns. The transition ends when the firing fraction reaches itsfinal target value at about 670 msec, denoted by line 206. The entiretransition length, the period between line 202 and 206, is approximately620 msec, during most of which time the cam phase is in motion. Alsoduring most or all of the transition time the firing fraction is higherthan required to generate requested torque. Typically, the air chargewould be managed throughout the transition to maintain the desiredcylinder torque output. This could be accomplished by reducing the MAPor MAF using any available actuator(s) such as throttle position.However, when necessary spark timing retard can be used as well. As willbe appreciated by those familiar with the art, both throttling and sparkretard tend to reduce fuel efficiency.

FIG. 2B illustrates another method of implementing a firing fractiontransition. The figure depicts changes in the cam phase angle 230 andthe firing fraction 232 as a function of time. Unlike the cam firstcontrol method depicted in FIG. 2A, the firing fraction and cam phaseangle begin their transition relatively contemporaneously once atransition is initiated at time 222, 50 msec. The firing fraction 232reaches its final target relatively quickly, while the cam phase 230continues at its maximum slew rate until it reaches its final targetvalue at time 226, approximately 635 msec. In this case, the totaltransition length is 585 msec, only slightly shorter than the casedescribed relative to FIG. 2A; however, the firing fraction is near itsfinal target value for much of that time. Again cylinder air charge(and, if necessary, spark retard) would preferably be controlled in anappropriate manner to ensure that the desired torque is providedthroughout the transition. This type of control is generally more fuelefficient, since MAP can be higher and/or spark more advanced than inthe case described relative to FIG. 2A. The duration of the transitionin the firing fraction is identical, since identical slew rates werechosen for the two cases.

FIG. 2C illustrates an exemplary transition that utilizes a staged, camfirst transition approach. The figure depicts the changes in the camphase angle 250 and the firing fraction 252 as a function of time. Thetransition begins at 50 msec, denoted by line 242. Immediately update acommand to change firing fraction the cam phase angle 250 begins tochange at its maximum slew rate, again in this example 60°/sec. The camphase angle changes at this maximum slew rate throughout the entiretransition until it reaches its final value of 15°. The firing fraction242 remains fixed at its initial value until the cam phase angle reachesa defined range about a first intermediate cam target value, occurringat about 90 msec denoted by line 248. In this example, the firstintermediate target value is a cam phase angle of 43° associated withthe first intermediate target firing fraction of ½. The firing fractionthen transitions to and remains at this level until the cam phase iswithin the target range of the next intermediate or final cam target. Inthis example, the next intermediate cam target is 29.5° associated withan intermediate firing fraction of ⅖ (see Table 1). The cam phase iswithin a range of this target value at 350 msec, denoted by line 247. Atthis time, the firing fraction begins a transition to the nextintermediated firing fraction level of ⅖. Once there the firing fraction252 remains fixed until the cam phase angle 250 reaches the definedrange of its final target value at approximately 580 msec, denoted byline 244. The firing fraction 252 then begins to transition towards itsfinal target value, in this example ⅓. The firing fraction slew rate inall transition steps is 0.004/msec, the same as in the previous twoexamples. It should be appreciated that the firing slew may not belinear and may have a different form and rate between all the firingfraction levels of the transition. The transition ends when the firingfraction reaches its final target value at about 635 msec, denoted byline 246. The entire transition length, the period between line 242 and246 is approximately 585 msec, the same as the transition lengthdepicted in FIG. 2B.

Referring next to FIGS. 3 and 4, several cam first control approaches inaccordance with various nonexclusive embodiments will be described. Inthe illustrated embodiment, the skip fire controller 10 (and moreparticularly transition adjustment unit 40) manages firing fractionchanges differently based on whether the request seeks to increase ordecrease the firing fraction. This distinction is represented by step302 in the flow chart of FIG. 3. When the change request seeks toincrease the firing fraction, more torque is typically being requestedso the transition to the new firing fraction begins substantiallyimmediately as represented by step 304 to help ensure that the requestedtorque can be delivered as quickly as possible. That is, the controllerdoes not wait for the newly requested cam position to be attained beforeinitiating the transition to a higher firing fraction. Nevertheless, thetransition to a higher firing fraction is typically implementedgradually over a short period of time in an effort to mitigate vibrationinducing torque fluctuations due to cam transition and manifoldfilling/emptying dynamics as described, for example, in the previouslyincorporated U.S. Pat. No. 9,086,020 (P011A) and U.S. Pat. No. 9,086,020(P029) and co-pending application Ser. No. 14/857,371 (P041), and62/296,451 (P054P). This is generally analogous to the approachillustrated above with respect to FIG. 2B.

When the change request seeks to lower the firing fraction, a cam firsttransition scheme may be employed. In this circumstance the controller10 (e.g., firing fraction & power train setting determining unit 30)selects a target firing fraction 33 (step 310) which in some instanceswill be an intermediate firing fraction that is between the currentfiring fraction and the requested firing fraction. The new target firingfraction may be the largest of the available firing fractions betweenthe current and final firing fraction, although this is not arequirement. The use of intermediate target firing fractions isparticularly useful when relatively larger firing fraction changes arerequested. Various methods of determining when and what values to selectas the intermediate firing fraction are described in more detail belowwith respect to FIG. 4.

After the (potentially intermediate) target firing fraction has beendetermined, the controller initiates a transition to a target cam phaseassociated with the target firing fraction in step 312. This cam phaseangle transition continues until the cam phase reaches—or is within apredetermined range of the target cam phase as represented by flow chartsteps 315 and 317. Once the cam phase has reached (or is close to) thetarget cam phase, a transition to the target firing fraction isinitiated in step 319. A variety of firing fraction transitionstrategies may be employed to minimize torques sags and/or surges duringthe firing fraction transition such as those described in theincorporated transition management patents—although again, the throttleplate position would typically be a primary mechanism used to manage aircharge through the firing fraction change. In some implementations, thetransition to the target cam firing fraction is completed before thetransition to the target firing fraction is initiated. In others, thetransition to the target firing fraction begins when the actual camphase approaches within a designated range of the target cam phase.

During the cam phase transition segment of the transition, the firingfraction is often held constant. Therefore, absent correction, the aircharge (and corresponding fuel charge) would change (typically increase)during the cam phase transition thereby resulting in an undesirabletorque surge. To avoid that problem the engine air charge is preferablyheld substantially constant through the cam phase transition bymodulating other parameters that affect air charge—e.g., the throttleposition or other actuators that impact the intake manifold pressure orthe mass air flow (MAF). To the extent that it is not possible tomaintain a substantially constant engine air charge, other operatingparameters (such as spark timing) can be modulated during the cam phasetransition to help avoid or minimize torque surges and/or sagsthroughout the transition. Here the term “substantially constant” doesnot limit engine air charge changes that might be necessary tocompensate for changes in engine efficiency and requested torque thatmay occur through the transition. In particular, the engine efficiencygenerally changes with firing fraction, which results in the totalengine air charge or mass air flow (MAF) changing to generate the sametorque. Therefore, over the course of the cam phase transition, thetransition adjustment unit instructs the CPU to modify certain operatingparameters (e.g. manifold pressure, spark timing etc.) in a manner thatmaintains the engine's torque output smooth through the transition. InFIG. 1, this is represented by requested manifold pressure signal 31′which may be modified relative to the manifold pressure 31 requested bypower train setting determining unit 30 during transitions.

After the cam phase transition has been completed (or is nearingcompletion), the transition from the previous firing fraction to thetarget firing fraction begins. Again, it is desirable to maintain asmooth torque output throughout the firing fraction transition segmentof the transition. Since changes in commanded air charge are realizedmore slowly than changes in firing fraction can be implemented, it isgenerally preferable to gradually adjust the firing fraction in a mannerthat corresponds to the change in air charge during this segment of thetransition as described in several of the incorporated firing fractiontransition management patents. Again, other variable engine parameterssuch as spark timing can also be used as necessary to maintain smoothtorque output throughout the transition.

In the discussion above, it is pointed out that the firing fractiontransition may optionally begin as the cam phase approaches the targetcam phase rather than always waiting for the target cam phase to beachieved. This is because relatively smaller changes in cam positionhave a relatively moderate impact on air charge. Accordingly, theoverall transition can be sped up by initiating the firing fractionchange before the target cam phase is actually reached.

In various preferred embodiments, during the firing fraction change, thecam phase preferably continues to transition along the same transitionpath towards the requested cam phase until the requested cam phase isreached. That is, the cam phase transition is not paused during thefiring fraction transitions. However, if desired for control purposes orotherwise, the cam phase may be held constant, or relatively constant,during the firing fraction transition with other variables such asthrottle position, manifold pressure, etc. being used as the primarymechanism to vary the air charge as required during the firing fractiontransition portion(s) of the overall transition. In embodiments thatbegin the firing fraction transition before the target cam phase isactually reached, the actual cam phase range within which the firingfraction change is initiated may vary with the needs of any particularapplication. However, by way of example, it has been found thatinitiating the firing fraction change when the actual cam phase iswithin approximately one to four degrees (1° to) 4° of the target camphase (which correlates to 2° to 8° of crankshaft rotation in a fourstroke engine) works well from a control standpoint.

Returning to the flow chart of FIG. 3, if the target firing fraction isthe requested firing fraction, the transition to the requested firingfraction continues as represented by step 324 until the transition iscompleted. In some cases, the cam angle may need to continue to movetowards its final target value in this block. In other cases, the finalcam target may be reached prior to attainment of the final target firingfraction. If the target firing fraction was an intermediate firingfraction, then the logic returns to step 310 where the same, cam firstlogic flow is then repeated until the requested firing fraction and aircharge/cam position are achieved.

Since the transition described above takes some time to complete, inpractice, the desired engine output (the requested engine torque), willsometimes (indeed often) vary to some extent due to changing drivingconditions or other factors. If the change in requested torque issignificant enough, it may trigger a change in the requested firingfraction during the course of a transition. The described processhandles any such changes quite well. If the requested firing fractionchanges to a value that is higher than the then current firing fraction,the skip fire controller 10 can begin transitioning to thenew—higher—firing fraction as soon as practical as represented by step302 in the flow chart. However, it should be appreciated that there isno need to wait until the then target firing fraction is attained beforeinitiating a transition to a higher firing fraction. Rather, firingfraction increase requests can be treated as an interrupt thatimmediately transitions the firing fraction control out of the flowillustrated in FIG. 3 to the immediate firing fraction increasetransition logic represented by step 304.

If the requested firing fraction changes to a value that is lower thanthe then current firing fraction, the process illustrated in FIG. 3 canreadily handle such change(s) using the same algorithm, simplysubstituting in the new value of the requested firing fractionregardless of whether it is higher or lower than the old requestedvalue.

During normal skip fire operation, when a change in requested torqueoccurs that is not large enough to trigger a change in firing fraction,the change in torque is typically met by changing the requested aircharge. This air charge change can be provided by changing the throttleblade position in concert with changes in cam phase angle. Generally thethrottle and cam may be adjusted to positions delivering maximum fuelefficiency. If necessary, spark timing may be adjusted appropriately aswell to help match the delivered with the requested torque. In somesituation where the torque request changes during a firing fractiontransition, a new cam target may be set. Such changes can also readilybe handled by the described process by simply using the “new” requestedcam phase as the requested cam phase.

There are a number of different processes that may be used to select thetarget firing fraction (step 310) for any iteration. One suitable methodwill be described next with reference to FIG. 4. As will be apparentfrom the following discussion, the method described with respect to FIG.4 incorporates a number of conceptual approaches that may be utilizedtogether or independently and/or in a variety of different combinations.

In the method of FIG. 4, the logic first determines whether the currentfiring fraction exceeds a predetermined threshold—which in theillustrated embodiment is one-half (step 336). If so, and the requestedfiring fraction is greater than or equal to the predetermined threshold(again ½ in the example), the target firing fraction is set to therequested firing fraction and a cam first transition approach is madedirectly to the requested firing fraction (following the flow 338, 341,312). Alternatively, if the current firing fraction exceeds thepredetermined threshold and the requested firing fraction is less thanthe predetermined threshold, then the target firing fraction is set tothat threshold (e.g., ½). Again a cam first transition strategy may befollowed to transition from the original firing fraction to theintermediate target firing fraction of ½ (338, 343, 319). For manyengines, a firing fraction of ½ is known to be a particularly smoothrunning firing fraction, and empirical evidence has shown that for someengines, stepping first to an intermediate firing fraction of ½ can workwell when relatively larger transitions that span the ½ threshold arerequested. Of course, different thresholds can be used when appropriatefor specific engines.

When the determination in step 336 is that the current firing fractionis less than or equal to the predetermined threshold, the logic flows tostep 351 where the controller obtains (or updates) a set of availablefiring fractions. The set of available firing fractions is a set offiring fractions that are deemed suitable for use to deliver the desiredengine output based on criteria designated by the engine controllerdesigner. Typically, that would be the set of firing fractions that arecapable of delivering the desired torque while still meeting desired NVHconstraints. Briefly, if the desired torque can be delivered byoperating ⅓ of the cylinders, then conceptually, any firing fraction ator above ⅓ is capable of delivering the desired engine output. However,some of these firing fractions may be undesirable for use at the currentoperating conditions (e.g., engine speed, gear, vehicle speed, etc.) dueto NVH or other concerns. Thus, not all of the firing fractions that arecapable of delivering the desired torque may be deemedappropriate/available for use at any particular time. In one example,achieving acceptable NVH tends to be more challenging at lower enginespeeds than higher engine speeds and at higher cylinder loads than atlower cylinder loads. Therefore, a particular firing fraction may besuitable for use when the engine speed is higher and the cylinder loadis lower, but not appropriate at lower engine speeds with a highercylinder load. The set of available firing fractions may be determinedusing lookup tables, algorithmically or using any other suitableapproach. By way of example, a few ways of determining available firingfractions are described in U.S. Pat. No. 9,086,020 (P011A) and U.S. Pat.No. 9,200,575 (P029) as well as U.S. patent application Ser. No.13/654,248 (P011B), Ser. No. 13/963,686 (P017), Ser. No. 14/638,908(P032) and Ser. No. 14/919,011 (P045B), each of which is incorporatedherein by reference.

For each of the currently available firing fractions, the cam phasesetting that would be appropriate for use at that firing fraction isdetermined as part of step 351. The appropriate cam phases can bedetermined using any appropriate approach. By way of example, lookuptables and/or various algorithmic or air model based approaches can beused to readily determine the appropriate cam phase settings for eachavailable firing fraction and cylinder load. The cam phase setting maycorrespond to that which yields the most fuel efficient operation at thedesired engine speed and cylinder load.

One way to pick the next target firing fraction is to look for thesmallest lower available firing fraction that has an associated camsetting within a predetermined range of the current cam position (or thecurrent target cam position). Step 354. The available firing fractionsare based at least in part on their NVH characteristics as previouslydescribed. The actual allowed range of cam settings that is appropriatewill vary based on a variety of factors, including cam phaserresponsiveness, emissions control, engine responsiveness, combustionstability, efficiency, design goals, etc. When desired, the acceptablerange may also vary as a function of current operating conditions, asfor example, engine speed, gear, road roughness, etc. By way of example,thresholds on the order of 2 to 8 degrees of camshaft rotation—as forexample 6 degrees of camshaft rotation may be appropriate in someembodiments. In such an implementation, the logic would look for thelowest firing fraction having an associated desired cam phase that iswithin the designated range (e.g. six degrees) of the cam phaseassociated with the current cam phase (or current target cam phase) and(if any such firing fractions exist) that firing fraction is selected asthe new target firing fraction as represented by flow chart step 357. Ifnone of the lower available firing fractions are within the prescribedrange, then the new target firing fraction is set to be the next lowestavailable firing fraction in step 361.

To further illustrate the staged, cam first transition strategydescribed above with respect to FIGS. 3 and 4, consider a request tochange the firing fraction from ⅔ to ⅓ (similar to the example shown inFIG. 2C) using a controller constrained to operate at firing fractionshaving a denominator of nine or less. In such a circumstance, any ofintermediate firing fractions ⅜, ⅖, 3/7, 4/9, ½, 5/9, 4/7, ⅗, ⅝ wouldpotentially be able to deliver the desired torque, although some may beexcluded from the available set due to NVH issues. In such an example,the set of potentially available firing fractions between the originaland requested firing fractions (inclusive), and their associated desiredcam phases at some engine speed and torque might look like Table 1.

TABLE 1 Firing Fraction Cam Advance (in crankshaft degrees) ⅔ 50°  ⅝49.5° ⅗ 48.5° 4/7 47.9° 5/9 46.9° ½ 43°  4/9 36.5° 3/7 34.3° ⅖ 29.5° ⅜24.6° ⅓ 15° 

As discussed above, not all of the intermediate firing fractions will bedeemed suitable for use all of the time. Therefore for the purposes ofexplanation, consider a circumstance in which the set of availablefiring fractions include 1, ⅘, ⅔, ⅗, ½. ⅖ and ⅓. In that scenario, theintermediate firing fractions ⅜, 3/7, 4/9, 5/9, 4/7, ⅗, ⅝ are excludedfor NVH or some other reason. In this example, the ultimately desiredcam advance is 15° (i.e., the cam advance associated with the requested⅓ firing fraction).

Since the original firing fraction was ⅔ and the requested firingfraction is ⅓, the first intermediate target firing fraction would be ½following steps 336, 338 and 343 of FIG. 4. The ½ target firing fractionhas a corresponding cam advance of 43°. Thus, following a cam firsttransition strategy, the camshaft begins transitioning towards therequested cam advance while maintaining the firing fraction at ⅔. Oncethe actual cam advance gets within a designated range of 43° (e.g.,within 3 degrees), a transition to the target ½ firing fraction is madeand the next target firing fraction is determined. As discussed above,the cam phase preferably continues to transition towards the ultimatelydesired cam advance of 15° during this intermediate firing fractiontransition, although that is not a requirement.

At this stage the only two lower firing fractions that are “available”are ⅖ and ⅓. Since, neither of these firing fraction have an associatedcam phase within a designated range of the current cam phase, the nextintermediate firing fraction would be set to the next lower availablefiring fraction (step 361), which in the example is ⅖, which has anassociated target cam phase of 29.5°. Using the cam first transitionapproach, the firing fraction is held at ½ until the actual cam phasecomes within the designated range of the new target cam phase at whichpoint a transition to the ⅖ firing fraction is initiated. In theexample, this transition may be initiated at about 32.5°. As discussedabove, the cam phase preferably continues to transition towards theultimately desired cam advance of 15° during this intermediate firingfraction transition, although that is not a requirement.

Repeating the same target selection process, the next target firingfraction would be the requested firing fraction of ⅓ since it is thenext lowest firing fraction. Continuing to use the cam first transitionapproach, the firing fraction is held at ⅖ until the actual cam phasecomes within the designated range of the new target cam phase at whichpoint a transition to the ⅓ firing fraction is initiated. In theexample, this transition may be initiated at about 18°. Since ⅓ is therequested firing fraction, the cam transition continues to the desiredcam advance of 15°.

In the embodiment illustrated in FIG. 4, three different approaches toselecting intermediate target firing fractions are described. Any ofthese approaches can be used independent of the others and/or incombination with any other defined intermediate target selectionschemes. Thus, for example, an alternate intermediate target selectionscheme could always transition to the next lower available firingfraction which would effectively amount to using illustrated step 361alone as the next target firing fraction selection criteria. If such anapproach was used in the example set forth above, the intermediatetargets would sequentially include ⅗, ½. ⅖ and ⅓ if those were the onlyavailable intermediate fractions. In other embodiments, the transitionselection logic could begin at step 351 with the next target selectionbeing determined based on the lowest firing fraction having anassociated cam phase within a defined range of a current camphase/target cam phase. Of course a wide variety of other targetselection strategies may be employed in other circumstances. Assuggested above, the defined cam phase change range allowed does notneed to be the same for all transitions. Rather, when desired thepermissible range at any time may vary based on the current firingfraction, current engine or vehicle operating parameters, or any otherparameters deemed important by the skip fire controller designer.

When no net torque is required a skip fire controlled engine with theability to close the intake and exhaust valves can disable all cylindersin a mode called decel cylinder cut off (DCCO) has described in U.S.patent application Ser. No. 15/009,533 (P048). In this mode the camphase exercises no control over engine operation, since the valves aredeactivated, i.e. closed. Consequently in entering and/or exiting DCCOno waiting is required for the cam to reach a target position.

In still other embodiments, transition tables could be provided thatpredefine the intermediate firing fractions that are used based onoperating conditions, the original and requested firing fractions and/orany other factors deemed important by the skip fire controller designer.Regardless of the selection process used, a significant point is thatintermediate target firing fractions can be used in some embodiments,which has the advantage of improving fuel economy during transitionsinvolving large cam phase changes compared to a single stage cam firstcontrol strategy.

Although the foregoing description focuses primarily on a staged, camfirst transition scheme that utilizes one or more intermediate targets,it should be appreciated that even single stage cam first transitioncontrol can advantageously be used in a variety of applications tosimplify the management of firing fraction transitions.

It should also be appreciated that the methods described herein relatedto coordination of the cam phase angle with firing fraction changesduring a firing fraction may be applied to other vehicle actuators inaddition to cam phase—although the specific control logic may vary basedon the nature of the particular actuator. For example, changes in thetorque converter slip (or more generally any adjustable drivetrain slipcomponent) can only be implemented relatively slowly, as for example, in½ to 2 seconds. Often to achieve acceptable NVH performance differentfiring fraction levels will have different amounts of torque converterslip. For example, torque converter slip may be relatively low at firingfractions of 1 or ½ and higher at firing fractions known to create moreNVH.

Unlike optimum cam phase angle, which tends to move monotonically withengine speed and torque request, desired driveline slip has a complexrelationship with respective to firing fraction, engine speed, andtorque request. Certain firing frequencies can excite vehicle resonancesor be particularly annoying to vehicle occupants. These firingfrequencies are generally avoided or use higher driveline slip levels toisolate vehicle occupants from the undesirable NVH. Other firingfrequencies may produce little undesirable NVH and in these cases slipcan be zero, i.e. locked-up TCC, or minimal.

Depending on the nature of the firing fraction transition and the sliplevels associated with the initial and final firing fraction levelsdifferent control methods may be used. A slip first method or a staged,slip first method, somewhat analogous to the cam first methods describedabove, may be used when the final slip level is higher than the initialslip level. A concurrent control method, analogous to the cam concurrentcontrol method described relative to FIG. 2B, may be used when the finalslip level is lower than the initial slip level and the firing fractionchange is small. Employing these various control strategies can improvefuel efficiency during a firing fraction while providing desirable NVHperformance. In some embodiments/circumstances, the torque converterslip may be varied simultaneous with changes in cam phase angle suchthat both the TCC slip and the cam phase and the actual firing fractiontransition (or firing fraction transition stage) is delayed until bothactuators are positioned adequately.

Referring next to FIGS. 5 and 6, a representative slip first transitionapproach will be described in more detail. In the illustratedembodiment, the logic flow begins when a firing fraction change requestis received as represented by step 500 of FIG. 5. In this embodiment,the requested firing fraction has an associated slip, which we refer toherein as the requested drivetrain slip. In general, the drivetrain slipmay be imparted by any drivetrain component that can have its slipactively controlled. Currently, torque converter clutches (TCC) are themost common commercially utilized drivetrain slip control mechanisms andtherefore the described embodiment focuses primarily on TCC slipcontrol. However, it should be appreciated that any other mechanism thatallows the controlled introduction of slip between an engine crankshaftand downstream power train components can be controlled in a similarmanner, as for example, an input clutch of an automated manualtransmission or a dual-clutch transmission, etc.

The magnitude of the requested operational slip will often vary based onthe requested firing fraction and optionally, a variety of otheroperating conditions or operating parameters, as for example, enginespeed, transmission gear, the magnitude of the torque request, cylinderload, vehicle speed, transmission input shaft speed, driver preferences,road roughness, environmental etc. The requested operational slip may bedetermined by firing fraction and power train setting determining unit30 in accordance with the policies and constraints of the vehicledesigner. In various embodiments, the appropriate slip values can be canbe obtained through the use of look-up tables, algorithmically or viaany other suitable approach.

One of the benefits of introducing relatively small amounts of slip tothe drivetrain during normal operation of a vehicle is that the sliptends to dampen engine generated vibrations, thereby reducing passengerperceptible NVH and smoothing the vehicle's ride. A drawback ofintroducing drivetrain slip is that it tends to slightly reduce fuelefficiency. Therefore, the actual slip values deemed appropriate fordifferent firing fractions and different operating conditions aretypically selected as a tradeoff between comfort and fuel efficiency.Thus, as a general rule, the more perceptible NVH a firingfraction/operating condition combination is likely to produce, thehigher the associated slip will be.

Returning to the flowchart of FIG. 5, the requested slip is compared tothe current slip in step 502. The current slip may be retrieved from theslip controller (e.g., a TCC controller) or any other component thatmaintains such information. Filtering may be provided to reduce noise onthe current slip signal. In some embodiments, a TCC controller may bearranged to broadcast or return a current slip level/group in additionto, or in place of the actual current slip value.

In general, the transitions to the requested operational slip and therequested firing fraction may begin substantially immediately if therequested operational slip is less than or equal to the current slip asrepresented by step 504. In some circumstances, it is also desirable tostart both transitions substantially immediately when the requested slipis higher, but close to the current slip—e.g., within a predeterminedrange of the current slip. That is, when the current (actual) slip isgreater than a slip transition threshold, both the firing fraction anddrivetrain slip transitions may start as represented by step 504. Thenature of the slip transition threshold may vary widely based on theneeds of any particular implementation. For example, in someembodiments, the slip transition threshold can simply be the requestedoperational slip. In other embodiments, the transition threshold can bea predetermined or a designated amount lower than the desiredoperational slip, which may permit the actual firing fraction change tobegin earlier in some operational states. By way of example, thedesignated slip threshold amount may be a fixed offset relative to thedesired operational slip, (as for example, being 5 or 10 rpm below thedesired operational slip). In other circumstances the difference betweenthe slip transition threshold and the desired operational slip may varybased on the magnitude of the operational slip and/or various otheroperating parameters. In various embodiments, the slip threshold may bea known constant offset relative to the operational slip, it may beobtained by reference to a lookup table, algorithmically or in any othersuitable manner.

Another relatively simple way to set the slip transition thresholds isto assign a “slip level” or “slip group” to each firing fraction. Forexample, the firing fraction or fractions having the best NVHcharacteristic can be assigned the lowest slip level (e.g., level 0),the next best set assigned the next lowest slip level (e.g., level 1)and so on. In such circumstances the logic only needs to determinewhether the requested operational slip has the same or lower slip levelthan the current slip level. Alternatively, each slip level cancorrespond to a designated range of slips, as for example 0-5 RPM, 5-15RPM, 15-25 RPM, etc.

The firing fraction change may be accomplished using a cam firsttransition approach as described above with respect to FIG. 3, bysimultaneously changing the firing fraction and air charge, or inaccordance with any other desired transition scheme—including, but notlimited to the transition schemes described in the various incorporatedpatents. It is noted that in many circumstances, the firing fractiontransition will complete quicker than slip transition—particularly whenthe requested slip is significantly less than the current slip. This canresult in operation in a condition where the actual slip is higher thanthe requested slip for a relatively brief period of time—which is finefrom a drivability standpoint because the NVH damping would always meetor exceed the design goal. Substantially the same logic can apply whenthe requested slip is a small amount higher than the current slip (i.e.,the requested slip is within a (relatively small) predetermined range ofthe current slip). In such circumstances there can be a small period inwhich the actual slip is potentially a small amount less than therequested slip. However, as long as the variance is relatively small, itmay be deemed acceptable by the system designers. It should also benoted that this same approach can be used regardless of whether thefiring fraction change request increases or decreases the operationalfiring fraction.

When the requested slip (or the slip transition threshold) is higherthan the current slip, a transition to the requested slip is initiatedas represented by step 510. In this condition, an immediate switch tothe requested firing fraction could cause the engine to operate for abrief period at the new firing fraction with a lower drivetrain slipthan specified. This can result in more perceptible NVH than desired. Tohelp mitigate this risk, the firing fraction change may be delayedand/or divided into multiple stages in a manner somewhat similar to theapproach discussed above for cam first transitions. As such, thetransition controller is arranged to identify intermediate target firingfractions that can be used at intermediate stages during the transitionas represented by step 512. The intermediate target firing fractions aregenerally limited to firing fractions capable of delivering the desiredengine torque while staying within their designated slip constraints. Afew intermediate target selection approaches are described below withrespect to FIG. 6.

In some cases, an intermediate target firing fraction candidate willhave an associated slip (slip transition threshold or slip level) thatis equal to or lower than the current slip. In such cases, thetransition to the intermediate firing fraction can initiate right away.In circumstances where all of the other available firing fractionsrequire higher slips (slip levels) than the current slip, a firingfraction with an intermediate slip can be chosen as the next target instep 512. While the slip is transitioning, the actual slip isperiodically compared to the target slip transition threshold (step 515,517). When the actual slip reaches the target slip transition threshold(i.e., reaches or gets close enough to the target slip) a transition tothe target firing fraction is initiated as represented by step 519. Ifthe target firing fraction is an intermediate firing fraction (asrepresented by the no branch of decision 521), the transition to therequested slip continues (522) and the logic returns to step 512 wherethe next target firing fraction is selected. The process then repeatsuntil the target firing fraction is the requested firing fraction (asrepresented by the yes branch of decision 521), at which point thetransition to the requested firing fraction completes (step 524) and thetransition is done.

Since the transition described above takes some time to complete, inpractice, the desired engine output (the requested engine torque), willsometimes (indeed often) vary to some extent due to changing drivingconditions or other factors. If the change in requested torque issignificant enough, it may trigger a change in the requested firingfraction during the course of a transition. The described processhandles any such changes quite well. The new firing fraction requesttriggers the process to restart (return to step 502) and the controllerbegins managing a transition from the then current state to the newlyrequested firing fraction.

There are a number of different processes that may be used to select thetarget firing fraction (step 512) for any iteration. One suitable methodwill be described next with reference to FIG. 6. As will be apparentfrom the following discussion, the methods described with respect toFIG. 6 incorporate a number of conceptual approaches that may beutilized together or independently and/or in a variety of differentcombinations and/or may be modified and/or combined with otherapproaches.

In the method of FIG. 6, the logic first determines whether therequested firing fraction is higher or lower than the current firingfraction (step 540). When the change request seeks to increase thefiring fraction, more torque is typically being requested. In suchcircumstances, it is often deemed important to almost immediatelytransition to a firing fraction capable of producing the requestedtorque so that the driver doesn't experience a power lag. As discussedabove, under any particular operating conditions, there will typicallybe a set of “available” firing fractions and associated powertrainsettings that can be used to deliver the desired torque while meetingthe designated NVH constraints. Most often, the requested firingfraction will be the lowest of the available firing fractions, and therewill very often be one or more other potentially available firingfractions—which would typically be higher than the requested firingfraction and include, at a minimum, an all cylinder operational mode,which corresponds to a firing fraction of 1.

When the requested firing fraction is higher than the current firingfraction (the yes branch of decision 540), the target firing fraction isset to the lowest available firing fraction having an associated sliptransition threshold that is at or below the current slip (i.e., has anassociated operational slip that is less than, equal to, or within adefine range of the current slip). (Step 542). It should be appreciatedthat when the requested firing fraction has a slip transition thresholdthat is higher than the current slip, the lowest available firingfraction having a slip at or below the slip transition threshold willoften (although not necessarily always) be a firing fraction that ishigher than the requested firing fraction. Accordingly, in such casesthe first intermediate firing fraction will be higher than the requestedfiring fraction and subsequent iteration(s) will move back downwardtowards the requested firing fraction in accordance with the logic ofFIG. 5. It should also be appreciated that when no other firingfractions are available that meet the slip transition thresholdrequirement, the first intermediate firing fraction can always be set toone (i.e., all cylinder operation)—which is expected to be associatedwith the lowest slip level in most implementations.

Also, the allowed slip for a firing fraction may depend on whether thestarting firing fraction is higher or lower than that firing fraction.So if the firing fraction is increasing, it may be desirable to tolerateless slip. Such control methods may be implemented using look-up tablesor determined algorithmically.

When the requested firing fraction is less than the current firingfraction (the no branch of decision 540), the logic determines whetherthere is an intermediate firing fraction (between the current andrequested firing fractions) that has an associated slip transitionthreshold that is less than or equal to the current slip (step 543). Ifsuch an intermediate firing fraction exists, then the target firingfraction is set to the lowest available firing fraction that has anassociated slip transition threshold that is at or below the currentslip (step 544). When there are no intermediate firing fractions thathave a slip transition threshold at or below the current slip, then theintermediate fraction having the lowest required slip threshold may beselected (step 549).

In the discussion above, efforts are sometimes made to identify thelowest available firing fraction having a suitable slip. Such firingfractions can be identified using any suitable algorithm. By way ofexample, in some implementations, a skip fire controller will maintain alist that identifies the set of firing fractions that are able toprovide the desired engine output within prescribed NVH criteria underthe current operating conditions. This list may be considered a list ofavailable firing fractions. When such a list of available firingfractions is used, the transition logic can first look at the slipthreshold associated with the lowest available firing fraction to checkwhether that slip threshold exceeds the current slip. If so, the nexthigher firing fraction is checked. If not, that firing fraction isselected as the target.

One side effect of increasing driveline slip is that it increasesdriveline losses, thereby reducing the power delivered to the wheels fora given engine output. Conversely, reducing driveline slip tend toincrease power delivered to the wheels for a given engine output. Whendesired, the engine controller can be configured to adjust the engineoutput in parallel with commanded driveline slip changes to partially orfully compensate for TCC or other driveline slip based variations in amanner that reduces or eliminates variations in the brake torquedelivered to the wheels during selected operating conditions.

Although a particular logic has been described to facilitate explanationof the invention, it should be appreciated that the actual algorithms orlogic used to accomplish the described functions may vary widely and arein no way intended to be limited to the logic flows illustrated in theaccompanying flow charts. Rather, various steps and functions may bereordered, altered, added or deleted in accordance with designerpreferences and/or the needs of any particular implementation.

Although only a few embodiments of the invention have been described indetail, it should be appreciated that the invention may be implementedin many other forms without departing from the spirit or scope of theinvention. For example, it should be appreciated that a variety of otheractuators can benefit from the same actuator first control approachesdescribed herein. Such actuators include (but are not limited to)various actuators associated with turbocharger or supercharger aircontrol such as an actuator that controls the position of a waste gateor tumble flap.

In the discussion above, cam first and slip first transition schemeshave mostly been described separately. However, it should be appreciatedthat combinations of slip first and cam first transition control canreadily be used. For example, the driveline slip and camshaft phase canbe adjusted as necessary in parallel and certain firing fractiontransitions (and particularly transitions to lower firing fractions) maybe delayed as necessary until both the slip and cam phase haveapproached or reached their associated targets.

As suggested above, it is generally desirable from a drivabilitystandpoint to maintain the torque output relatively smooth duringtransitions. In the described cam first transition approach, there aretypically two distinct sections associated with any transition. Onefocuses on transition of the cam phase, the second focuses on transitionof the firing fraction. The goal is to make both sections of thetransitions smooth so it is generally desirable to utilize othervariables to control the overall torque output during each section ofthe transition.

When the requested engine output stays constant throughout a transition,it is generally desirable to maintain a relatively constant air chargethroughout the cam phase transition. In many applications the cam phaseand a throttle (to control the manifold pressure) are the primarymechanisms to control/vary the air charge in each cylinder. Therefore,as the cam phase changes, it is desirable to vary the manifold pressurein a complementary way to maintain a relatively constant air chargethrough the cam phase change. Of course, there are a variety of otherways to vary the air charge as well including valve lift control,exhaust gas recirculation techniques, air boosting techniques such asturbo-charging, supercharging, etc. When the engine includes suitablehardware, any of these air charge control mechanism can be controlledindividually, or in parallel to help control the air charge.

Once the desired cam phase has been attained (or is close) the firingfraction is changed. Although it is possible to change the firingfraction almost instantaneously, it is typically not possible to changethe air charge as quickly. Therefore, the firing fraction is typicallychanged over a transition period in a manner that tracks air chargedynamics, which again may be controlled using the throttle or otheravailable mechanisms.

Some skip fire controllers are arranged such that they will inherentlyinvoke a relatively large number of transitions under a variety ofnormal driving scenarios in an effort to maximize fuel economy. This isparticularly true in driving conditions that support a relatively largeset of firing fractions. By way of example, some driving tests byApplicants of a skip fire controller having up to 29 available firingfractions tend to average a transition every second or two duringvarious normal driving profiles. In these cases an engine may operateapproximately ⅕ of the time in a transition between firing fractionlevels. To meet required driving comfort while improving fuel economy,this makes it particularly desirable to utilize some of the transitionmanagement approaches described herein.

In the foregoing description, there are several references to the term,“cylinder.” The term cylinder should be understood as broadlyencompassing any suitable type of working chamber. The figuresillustrate a variety of devices, designs and representative cylinderand/or engine data. It should be appreciated that these figures areintended to be exemplary and illustrative, and that the features andfunctionality of other embodiments may depart from what is shown in thefigures.

The invention has primarily been described in the context of dynamicskip fire operation in which an accumulator or other mechanism tracksthe portion of a firing that has been requested, but not delivered, orthat has been delivered, but not requested. However, the describedtechniques are equally applicable to managing transitions between anydifferent skip fire firing fractions or between a skip fire firingfraction (in which individual cylinders are sometimes fired and sometimeskipped) and all cylinder operation (or operation using a fixed set ofcylinders) as may occur when using various rolling cylinder deactivationtechniques. Similar techniques may also be used to manage effectivedisplacement transitions in variable stroke engine control in which thenumber of strokes in each working cycle are altered to effectively varythe displacement of an engine.

The present invention may also be useful in engines that do not use skipfire control. For example, although the invention is described primarilyin the context of transitions between different firing fractions duringskip fire control, the described techniques can also be used tofacilitate transitions between different variable displacement states inmore traditional variable displacement engines using a skip firetransition approach. For example, an eight cylinder variabledisplacement engine that has the ability to operate in a 4 cylinder mode(i.e., 4 fixed cylinders) will require transitions from a firingfraction of 0.5 to 1 and vice versa and could advantageously use thefiring fraction transition management techniques described herein.Therefore, the present embodiments should be considered illustrative andnot restrictive and the invention is not to be limited to the detailsgiven herein.

1. A method of managing firing fraction transitions in a vehicle havinga powertrain, the powertrain including an engine and a drivetrain, thedrivetrain including an adjustable slip drivetrain component, the methodcomprising: while the engine is operating at a first firing fraction andthe adjustable slip drivetrain component is operating at a firstdrivetrain slip, determining a requested second firing fraction that isdifferent than the first firing fraction, the second firing fractionhaving an associated second drivetrain slip and an associated seconddrivetrain slip transition threshold that is higher than the firstdrivetrain slip; initiating a transition from the first drivetrain sliptowards the second drivetrain slip; transitioning to a target firingfraction that is different than the first and second firing fractions,the target firing fraction (i) being selected from a set of availablefiring fractions capable of delivering a requested engine output, and(ii) having an associated target drivetrain slip transition thresholdthat is less than the second drivetrain slip transition threshold; andafter transitioning to the target firing fraction, transitioning to thesecond firing fraction, and wherein each firing fraction transition isconstrained to only occur when an actual driveline slip is at least ashigh as the associated drivetrain slip transition threshold.
 2. A methodas recited in claim 1 wherein when the requested second firing fractionis higher than the first firing fraction, the selected target firingfraction is the lowest available firing fraction capable of deliveringthe requested engine output that has an associated driveline sliptransition threshold that is not more than the first drivetrain slip. 3.A method as recited in claim 1 wherein when the requested second firingfraction is lower than the first firing fraction, the selected targetfiring fraction is an intermediate firing fraction between the first andsecond firing fractions.
 4. A method as recited in claim 3 wherein theselected target firing fraction is the lowest available firing fractioncapable of delivering the requested engine output that has an associateddriveline slip transition threshold that is not more than the firstdrivetrain slip.
 5. A method as recited in claim 3 wherein the selectedtarget firing fraction has an associated drivetrain slip transitionthreshold between the first driveline slip and the second driveline slipthreshold.
 6. A method as recited in claim 1 wherein the adjustable slipdrivetrain component is a torque converter clutch (TCC).
 7. A method asrecited in claim 1 wherein: an engine controller that directs the firingfraction transitions has a defined set of potential operational firingfractions; at various particular operating conditions, only anassociated subset of the potential operational firing fractions aredesignated as candidates for use; the firing fraction candidates for usedo not always include all of the potential operational firing fractionsthat are capable of delivering the requested engine output; and thetarget firing fraction is selected from the firing fraction candidatesfor use at current operating conditions.
 8. A method as recited in claim1 wherein during each firing fraction transition, a commanded air chargeis changed during the transition in a manner that helps provide adesired engine output throughout the transition.
 9. A method as recitedin claim 1 wherein when the requested firing fraction changes during atransition to a new requested firing fraction, the adjustable slipdrivetrain component is commanded to begin transitioning to a drivelineslip associated with the new requested firing fraction.
 10. A method asrecited in claim 1 wherein the set of available firing fractions onlyincludes fractions having a denominator that is not greater than nineand a firing fraction of one.
 11. A method of controlling an engine in avehicle having a powertrain, the powertrain including the engine and adrivetrain, the drivetrain including an adjustable slip drivetraincomponent, the method comprising: while the engine is operating at afirst firing fraction having an associated first drivetrain componentslip, determining a desired second firing fraction and an associatedsecond drivetrain component slip, the second drivetrain component slipbeing different than the first drivetrain component slip; initiating atransition from the first drivetrain component slip to the seconddrivetrain component slip; after an actual drivetrain component slip isat or within a predetermined range of the second drivetrain componentslip, transitioning to the second firing fraction, wherein thetransition to the second firing fraction is initiated only after theactual drivetrain component slip is at or within the predetermined rangeof the second drivetrain component slip; and directing operation of theengine during the transitions to deliver a requested output, whereby anactual commanded firing fraction gradually changes over the course ofthe transition from the first firing fraction to the second firingfraction.
 12. A method of controlling an engine in a vehicle having apowertrain, the powertrain including the engine and a drivetrain, thedrivetrain including an adjustable slip drivetrain component, the methodcomprising: (a) while the engine is operating at a first firing fractionand the adjustable slip drivetrain component is operating at a firstdrivetrain slip, determining a requested second firing fraction that isdifferent than the first firing fraction, the second firing fractionhaving an associated second drivetrain slip, the second drivetrain slipbeing higher than the first drivetrain slip; (b) initiating a transitionfrom the first drivetrain slip to the second drivetrain slip; (c)identifying a target firing fraction that is different than the firstand second firing fractions from a set of available firing fractionscapable of delivering a requested engine output, the target firingfraction having an associated target drivetrain slip that is less thanthe second drivetrain slip; (d) transitioning to the target firingfraction, wherein the transition to the target firing fraction isconstrained to only occur when an actual driveline slip is at least ashigh as a target driveline slip threshold associated with the targetfiring fraction; and (e) after transitioning to the target firingfraction, transitioning to the second firing fraction, wherein thetransition to the second firing fraction is constrained to only occurwhen the actual driveline slip is at least as high as a second drivelineslip threshold associated with the second firing fraction; and (f)directing operation of the engine during the transitions to deliver therequested engine output.
 13. A method as recited in claim 12 furthercomprising: (a) after transitioning to the target firing fraction,transitioning to a next target firing fraction from the set of availablefiring fractions capable of delivering the requested engine output, thenext target firing fraction having an associated driveline slipthreshold that is higher than the target driveline slip thresholdassociated with the target firing fraction and no more than the seconddriveline slip threshold, wherein the transition to the next targetfiring fraction is constrained to only occur when an actual drivelineslip is at least as high as the next target driveline slip threshold;and (b) iteratively repeating (a) until the second firing fraction isreached.
 14. A method as recited in clam 12 wherein the adjustable slipdrivetrain component is a torque converter clutch (TCC).
 15. A method ofcontrolling an engine in a vehicle having a powertrain, the powertrainincluding the engine and a drivetrain, the drivetrain including a torqueconverter clutch (TCC) the method comprising: (a) while the engine isoperating at a first firing fraction and the torque converter isoperating at a first slip, determining a requested firing fraction thatis lower than the first firing fraction, the requested firing fractionbeing a second firing fraction having an associated second torqueconverter slip, the second torque converter slip being higher than thefirst torque converter slip; (b) initiating a transition from the firsttorque converter slip to the second torque converter slip; (c) selectingan intermediate firing fraction that is between the first and secondfiring fractions, the intermediate firing fraction having an associatedtorque converter slip; (d) identifying the selected intermediate firingfraction as a target firing fraction; (e) transitioning to the targetfiring fraction, wherein the transition to the target firing fraction isconstrained to only occur when an actual torque converter slip is atleast as high as, or within a predetermined range of the torqueconverter slip associated with the target firing fraction; (f) when thetarget firing fraction is not the second firing fraction, selecting anext firing fraction having an associated next torque converter slip andsetting the target firing fraction to be the next firing fraction; (g)iteratively repeating (e) and (f) until the requested firing fraction isreached; and (h) directing operation of the engine during thetransitions to deliver a requested output.
 16. A method as recited inclaim 15 wherein the intermediate firing fraction is determined byselecting a lowest firing fraction candidate that has an associatedtorque converter slip that is no greater than or within a predefinedrange of the first torque converter slip at current operatingconditions.
 17. A method as recited in claim 16 wherein the next firingfraction is determined by selecting the lowest firing fraction candidatethat has an associated torque converter slip within a predefined rangeof a then current target torque converter slip.
 18. A method as recitedin claim 15 wherein: an engine controller that directs the firingfraction transitions has a defined set of firing fraction candidates;each target firing fraction is selected from the firing fractioncandidates for use at current operating conditions.
 19. A powertraincontroller arranged to direct skip fire operation of an engine and todirect a slip setting of a drivetrain component, the powertraincontroller being configured to: direct operation of the engine at afirst firing fraction and direct operation of the drivetrain componentat a first slip; while the engine is operating at the first firingfraction and the adjustable slip drivetrain component is operating atthe first drivetrain slip, determine a requested second firing fractionthat is different than the first firing fraction, the second firingfraction having an associated second drivetrain slip and an associatedsecond drivetrain slip transition threshold that is higher than thefirst drivetrain slip; initiate a transition from the first drivetrainslip towards the second drivetrain slip; direct transition to a targetfiring fraction that is different than the first and second firingfractions, the target firing fraction (i) being selected from a set ofavailable firing fractions capable of delivering a requested engineoutput, and (ii) having an associated target drivetrain slip transitionthreshold that is less than the second drivetrain slip transitionthreshold; and after directing transition to the target firing fraction,direct transition to the second firing fraction, wherein each firingfraction transition is only directed when an actual driveline slip is atleast as high as the associated drivetrain slip transition threshold.20. A powertrain controller as recited in claim 19, wherein thepowertrain controller has a defined set of potential operational firingfractions; at various particular operating conditions, only anassociated subset of the potential operational firing fractions aredesignated as candidates for use; the firing fraction candidates for usedo not always include all potential operational firing fractions thatare capable of delivering the requested engine output; and each targetfiring fraction is selected from the firing fraction candidates for useat current operating conditions.
 21. A powertrain controller arranged todirect skip fire operation of an engine and to direct a slip setting ofa drivetrain component, the powertrain controller being configured to:direct operation of the engine at a first firing fraction and operationof the first drivetrain component at a first drivetrain component slip;while the engine is operating at the first firing fraction and the firstdrivetrain component is operating at the first drivetrain componentslip, determine a desired second firing fraction and an associatedsecond drivetrain component slip, the second drivetrain component slipbeing different than the first drivetrain component slip; initiate atransition from the first drivetrain component slip to the seconddrivetrain component slip; after an actual drivetrain component slip isat or within a predetermine range of the second drivetrain componentslip, direct a transition to the second firing fraction, wherein thetransition to the second firing fraction is initiated only after theactual drivetrain component slip is at or within the predetermined rangeof the second drivetrain component slip; and direct operation of theengine during the drivetrain component slip and firing fractiontransitions to deliver a requested output.